The invention relates to heat-resistant positive resists based upon precursor stages of highly heat-resistant polymers and light-sensitive diazoquinones, as well as a method for the preparation of heat-resistant relief structures of such positive resists.
Heat-resistant and highly heat-resistant photo resists are required in a number of applications, particularly for the modern processes of structuring and doping of semiconductors, i.e., for dry-etching processes such as plasma etching or reactive ion etching, and for ion implantation. The resist materials and the corresponding relief structures made therefrom must not decompose at the high temperatures of such processes, and deviations or alterations in the dimensions of the relief structures, for example, by softening or distortion, also must not occur.
Conventional positive resists, such as those of the Novolake type, meet these increased thermal requirements only in part, i.e., they have limited dimensional heat stability. This, however, impairs the reliability of the processes in which these resists are used and adversely affects the advantages of the process such as steepness of the edges, fine structure, high etching and doping rate, and the like.
Heat resistant negative resists are known from U.S. Pat. Nos. 3,957,512; 4,045,223; 4,088, 489 and Re. 30,186. While these negative photo resists are highly suitable for the preparation of highly heat-resistant relief structures, they are subject to the disadvantages which are inherent in negative resists. For, as compared to negative resists, positive resists are distinguished particularly by high resolution, short exposure times, the ability to utilize aqueous-alkaline developers, which is of importance ecologically and economically, and by the fact that the presence of oxygen has no effect on the exposure time.
From U.S. Pat. No. 4,093,461, a heat-resistant positive resist is known which comprises a light-sensitive o-quinone-diazide (o-diazoquinone) or o-naphthoquinone-diazide (o-diazonaphthoquinone) and a polyamido carboxylic acid in the form of a polycondensate product of an aromatic dianhydride and an aromatic diamino compound; the polyamido carboxylic acid, called "polyamic acid" in the mentioned patent, is a polymer precursor stage and more specifically, the polymer precursor stage of a polyimide.
However, it has now been found that this known positive resist composition has only limited storage life, because the diazoquinones exhibit only a very limited storage stability in the presence of acids. In addition, the stability of the mentioned photo resist to alkaline etching solutions is insufficient. Furthermore, the differences in the solubility between the exposed and the unexposed portions of the photo resist are relatively small.